This invention relates to cutting means for an underwater plastic pelletizer, and more particularly to improved means for cutting and transferring plastic pellets from extruded plastic polymers.
Heretofore, such as shown in U.S. Pat. No. 4,300,877 dated Nov. 17, 1981 underwater plastic pelletizers have been provided in which hot plastic material extruded through the orifices of a die plate into a water filled housing are cut into short lengths or pellets by rapidly rotating knives adjacent a side of the die plate. The pellets are cooled by the water and carried from the water filled housing to a collection station.
A rotor is mounted adjacent a face of the die plate from which the plastic material is extruded and a plurality of circumferentially spaced cutting blades or knives are secured to a hub of the rotor for rotation therewith. The cutting edges of the blades normally ride along or in closely spaced relation to the surface of the die plate during rotation of the rotor. By including or tilting the blade at a tilt angle of around 45.degree. for example with respect to the adjacent face of the die plate, a self sharpening cutting edge is provided as the cutting edge positioned at the same angle as the tilt angle. Also, a negative rake angle is provided so that a cut pellet is effectively pulled away from an extruded plastic strand. As the blades normally wear unevenly, the blades will not always be equally spaced from the die plate since mounted in a fixed relation on the rotor or rotor hub.
U.S. Pat. No. 4,251,198 dated Feb. 17, 1981 is likewise directed to an underwater thermoplastic pelletizer including a rotor hub with blades extending therefrom and having cutout portions between the blades. The blades are rigidly secured to the hub and have opposed cutting edges for reversal of the blades to provide a new cutting edge upon wear of the opposed cutting edge. The rotor and hub are normally urged toward the face of the die plate at a predetermined loading to urge the cutting edge into contact with the die plate. As the blades are fixed to the hub, an uneven wear will normally occur on the blades. The open areas of the hub in U.S. Pat. No. 4,251,198 in combination with the configuration of the blades permit the water and pellets in the underwater pelletizer to flow more freely to minimize vortexing of the water resulting from the centrifugal water flow generated by the rapidly rotating hub and blades.
As an example of a pelletizer in which the axial position of the rotor hub and blades thereon relative to the die plate may be controlled and maintained at a predetermined distance, reference is made to U.S. Pat. No. 4,529,370 dated July 16, 1985. A predetermined distance between the cutting blades and die plate is maintained by sensing the conductivity between the die face and cutter assembly carrying the blades which is imputed to a microcomputer for effecting axial movement of the cutter assembly or hub toward the die plate upon a wearing of the blades thereby to maintain a precise predetermined distance between the blades which are fixed to a hub and the die plate. The cutting edges of the blades are self sharpening.